refactor: remove redundant test assertions flagged in code review

Agent-Logs-Url: https://github.com/Azure/apiops-cli/sessions/8b196bc2-81c8-4320-9a60-c3655ee4b3bc

Co-authored-by: petehauge <12090616+petehauge@users.noreply.github.com>

Author: Copilot

tests/unit/lib/user-agent.test.ts

@@ -8,8 +8,6 @@ import { USER_AGENT } from '../../../src/lib/user-agent.js';
 describe('USER_AGENT', () => {
   it('should be a string', () => {
     expect(typeof USER_AGENT).toBe('string');
-    expect(USER_AGENT).toBeDefined();
-    expect(USER_AGENT).not.toBeNull();
   });
 
   it('should start with "apiops-cli/"', () => {

tests/unit/services/keyvault-checker.test.ts

@@ -629,7 +629,6 @@ describe('defaultArmRequest User-Agent header', () => {
       const [_url, init] = call as [string, RequestInit];
       const headers = new Headers(init?.headers);
       expect(headers.get('User-Agent')).toBe(USER_AGENT);
-      expect(headers.get('User-Agent')).toMatch(/^apiops-cli\/\d+\.\d+\.\d+/);
     }
   });
 });